winpr: fix win32 linking issues

winpr: fix win32 linking issues

Depending on the windows target version (_WIN32_WINNT), the used
SDK and the build configuration the linker will see multiple
libraries exporting the same symbols.
To prevent ugly hacks (e.g. modifying cmake's default system
libraries or fragile library linking order chains) we prefix
these functions with "winpr_" and create corresponding defines
to keep the current api names.